Plastic materials are being used in a number of automotive engineering applications to enhance vehicle styling. For example, plastic materials are currently used in the manufacturing of such parts and components as B-pillars, headlamps, and sunroofs. An emerging application for transparent plastic materials is automotive window systems. When a transparent plastic material is used to manufacture an automotive window, regulatory requirements call for the window to include some form of identification marking or logo. In addition, manufacturer's find it desirable for the automotive window to include an opaque fade-out border in order to enhance the overall appearance of the installed window. Finally, in order for plastic windows to be durable over a period of years, the window is covered by a coating that will provide the window with protection against weathering and scratches/abrasion.
In order to effectively mark a plastic window with an identification marking and a fade-out border, inks that are used must not only adhere to the surface of the plastic window, but also must be compatible with the protective coating system that is applied to the window's surface. Any ink used to mark the surface of a plastic window must not be softened, damaged, or removed during the application of the protective coating system. In addition, the inks need to be able to survive the rigorous testing required to qualify the product by the automotive industry.
The use of a UV curable ink to mark a plastic window offers a key advantage over thermally cured ink systems. Primarily, the use of a UV ink can increase productivity by decreasing the amount of time required to cure the ink. A conventional ink can take anywhere from 15 to 60 minutes to thermally cure, while a UV ink will cure within a matter of seconds. A further improvement in productivity can be obtained by reducing the cycle time required to remove “dried” ink residue left in the screen during the screen printing of a thermally curable ink. This “dried” ink residue arises from the evaporation of the solvent(s) in the thermally curable ink. The removal of a screen from the printing operation and replacing it with another screen and fresh ink can take upwards to an hour of time. The elimination of this cleaning operation along with the dramatic decrease in time required to cure a UV ink provides for substantial productivity gains.
The major problem associated with using a UV curable ink in a plastic glazing panel for an automotive window, is that typical commercially available UV inks are not compatible with the application of a protective coating system. The adhesion at the interface between the ink and the coating system or between the ink and the plastic window is weakened during the lifetime of the window, resulting in premature delamination or microcracking of the ink and/or coating system. One contributing factor to the occurrence of this phenomenon is that the inks applied need to be substantially opaque in nature. The complete curing of an opaque ink by the application of UV radiation onto the surface of the printed ink is difficult to accomplish. The UV radiation either becomes absorbed or is reflected upon interacting with the surface layer of the opaque ink, thereby, reducing or only partially curing the bulk of the ink; especially the ink located near the interface established with the plastic's surface.
Therefore, there is a need in the industry to be able to manufacture plastic glazing panels with a high level of productivity using a UV ink that is substantially curable and compatible with the application of a protective coating system.